1. Field of the Invention
The invention relates to the pelletizing of plastics and, more particularly, to a novel improvement for achieving, e.g., a longer life of a cutter knife, a biasing force exerted thereon is adjusted by a pressure medium.
2. Related Art
A variety of pelletizer for plastics have conventionally been used. A typical one is disclosed in U.S. Pat. No. 5,190,768 whose construction is shown in FIG. 3.
In FIG. 3 reference numeral 1 designates a pelletizer, which is secured to a die surface 3a side of a die 3 through a cutter case 4 which houses a cutter 5. The die 3 is mounted on a manifold 2 so as to be in intimate contact therewith, the manifold 2 distributing molten resin extruded from an extruder (not shown) in the circumferential direction.
The die 3 is of the disk-like shape and has many nozzles 6, 6, . . . axially passing through the die in a ring-like area encircling the center of the die 3. One end of each nozzle 6 is opened toward a ring-like resin passage 2a of the manifold 2, and the other end thereof is opened within the cutter case 4.
One side of the cutter case 4 is releasably secured to the surface 3a side of the die 3 by bolts 7, and the pelletizer 1 is secured to the other side thereof by bolts 53. A cooling water inlet 8 is arranged at the lower portion of the cutter case 4, and a cooling water outlet 9 is arranged at the upper portion thereof, so that the cutter case 4 is filled with cooling water at all times.
In the pelletizer 1 is a sleeve 52. The sleeve 52 is contained in a housing 15 through an O ring 50 and a plurality of packings 51 so as to be movable by a predetermined distance in the axial direction. Within the sleeve 52 is a cutter drive shaft 12. The cutter drive shaft 12 is held through a pair of shaft sealing members 54 and a pair of bearings 55 so as to be rotatable coaxially and immovable in the axial direction (longitudinal direction) relative to the sleeve 52.
In cutter drive shaft 12, the cutter holder 10 is held at an end portion of the cutter drive shaft 12 projecting toward the inside of the cutter case 4 through a bolt 10a and a key 10b so as to be rotatable integrally with the cutter drive shaft 12 and releasable. A driving unit (not shown) is connected to the other end of the cutter drive shaft 12 through a shaft coupling (not shown) that is slidable in the axial direction.
On an end surface of the cutter holder 10 confronting the die 3 is a plurality of cutter knives 11, 11, . . . radially held. The cutter knives 11 are located so as to confront the die surface 3a of the die 3, i.e., the surface of the ring-like area on which the nozzles 6, 6, . . . are arranged.
The sleeve 52 forms a piston cylinder between a rear portion 15a of the housing 15 together with a rear cover 63 and a rear portion 52a of the sleeve 52 serving as a piston. A first gap chamber 56, which have a stroke value S, and a second gap chamber 57 formed on both sides of the rear portion 52a have a first hole 58 and a second hole 59, respectively, the holes 58, 59 communicate with the first gap chamber 56 and the second gap chamber 57, respectively. That is, the cutter drive shaft 12, the sleeve 52, and the housing 15 constitute a mechanism 100 for axially moving the cutter knives 11.
In the rear portion 52a of the sleeve 52 is a key 60. The key 60 that extends in the radial direction is projected outward while passing through a through groove 61 formed in the rear portion 15a of the housing 15. A detecting rod 62a of a detecting means 62 implemented by, e.g., a dial gage disposed on an outer surface 15b of the housing 15 is abutted against the key 60.
The ring-like rear cover 63 is attached to a rear end 15c of the housing 15 through bolts 64, and a stopper 66 having operation levers 65 is screwed to a screwing portion 63a formed on an inner diameter portion of the rear cover 63.
The stopper 66, when rotated to move in the axial direction, abuts against an adaptor 67 coupled to a rear end 52c of the sleeve 52 at a predetermined position, so that the stopper operation in the axial direction can be obtained.
Further, the first hole 58 is connected to a pressure medium source 69 through a first switching valve 70 and a first pressure adjusting valve 68, the pressure medium source 69 having a pressure medium such as compressed air or pressure oil. The second hole 59 is connected to the pressure medium source 69 through a second switching valve 72 and a second pressure adjusting valve 71. To monitor the pressure of the pressure medium supplied to the first hole 58 and the second hole 59, a pressure gage 73 is connected to each of the pressure adjusting valves 68, 71.
The conventional pelletizer for plastics is constructed as described above. A method of pelletizing plastics will now be described.
First, when the second switching valve 72 and the first switching valve 70 are set to the supply position and the discharge position, respectively, the sleeve 52 moves toward the die 3 within the housing 15 by the same action as the piston by the pressure medium supplied to the second gap chamber 57, biasing the cutter knives 11 onto the die surface 3a. This condition is set to zero at the detecting means 62.
When the cutter drive shaft 12 is rotated by the driving unit (not shown) under the above-mentioned condition, the cutter knives 11 are rotated at a high speed, causing the molten plastic (not shown) extruded out of the nozzles 6 to be cut into pellets by the respective cutter knives 11. The cut pellets are solidified by the cooling water supplied into the cutter case 4 from the cooling water inlet 8, and sent to a processing section (not shown) from the cooling water outlet 9 together with the cooling water. The pellets are dried and made into products at the processing section.
When the above-mentioned cutter knives 11 rotate within water, a thrust directed toward the die 3 (advancing thrust) is generated. While this thrust is changed in proportion to the number of rotations of the cutter knives 11, the thrust takes a significantly large value at normal high rotational speeds. Particularly, it is noted that in recently used large-capacity pelletizers 1 the cutter knives 11 tend to be operated at high rotational speeds, so that the generation of the thrust leads to an increase in the biasing force of the cutter knives 11 onto the die surface 3a, wearing the cutter knives 11 drastically.
Thus, with respect to the advancing thrust generated by the rotation of the cutter knives 11, the first switching valve 70 is switched to the supply side to supply the pressure medium to the first gap chamber 56, so that a force in such a direction as to cause the sleeve 52 to retreat against the thrust is exerted on the sleeve 52 to cancel out most of the advancing thrust.
That is, the setting of the first pressure adjusting valve 68 is set to a level higher than that of the second pressure adjusting valve 71 to cancel out most of the thrust, and the respective pressure adjusting valves 68, 71 are set so that the cutter knives 11 can bias the die surface 3a with a desired force with the remaining advancing thrust.
As a result of the above-mentioned operation, the cutter knives 11 are rotated while biased onto the die surface 3a with a desired weak biasing force at all times, allowing the molten plastic to be cut.
Further, how much each cutter knife 11 is worn can be observed from outside the pelletizer by reading the graduation on the detecting means 62.
Since the conventional pelletizer for plastics is constructed as described above, the following problems have been addressed.
In the conventional construction, when the molten resin exiting from the die is cut, the edge of each cutter knife is worn. Therefore, to ensure stable cutting, each cutter knife must always form a new edge by causing itself to come in contact with the die surface and to wear the edge thereof. However, the wearing speed of the cutter knife depends on the kinds of resins cut. As for some resins with which the wearing speed of the cutter knife is slow, in such a case, it is possible to slow the wearing speed and/or the cutter knife is not always biased in a period of some time.
However, with the above-mentioned conventional structure the thrust that biases the cutter knives onto the die surface is exerted at all times, so that the biasing of the cutter knives cannot be stopped.
In addition, in the above-mentioned conventional construction, the die surface and the cutter knives are brought into contact with each other by an appropriate biasing force at all times, and the cutter knives are rotated under this condition. Therefore, the die and the cutter knives must be made of excellent wear resistant materials. For example, an expensive material such as TIC (titanium carbide) is used. If the cutter knife is made of an inexpensive material such as stainless steel or machine tool steel, then the life of the cutter knife is shortened, and if the cutter knife biasing force is reduced to increase the life of the cutter knife, the cutter knife is bounced back by the resin extruded out of the die. This gives a gap between the die surface and the cutter knives, leading to defective cutting.
Further, the pressure of the cooling water within the cutter case is sometimes changed drastically (e.g., when the three-way valve in a cut pellets transport pipeline is switched). In such a case, a force in the retreating direction is exerted on the sleeve end surface exposed to the cutter case by the water pressure elevated at such instance to bounce back the cutter knives, which in turn forms a gap between the die surface and the cutter knives to cause defective cutting. Thus, in the case where a drastic change in the water pressure is likely to happen, it is necessary to set the pressure of the pressure medium to be supplied to the gap chamber for adjusting the advancing thrust to such a level as to increase the advancing thrust in advance. To increase the advancing thrust means that the cutter knives are biased onto the die surface with a force larger than necessary, and this accelerates the wearing speed of the cutter knives.